Using different FX's

I don't understand the ReShade thing. I opened up the folder, and ran Mediator.exe as told. I added the GTA.exe file, and selected Direct3D 10/11, and clicked add. I then loaded up the game. It worked. It looked different.
But I do not understand the second section on the Mediator file. What am I supposed to do with it?
Also, I see there are different FX's. Such as BasicDX, GemFX, McFX, SweetFX. I also saw folders and files for each of these. What are these? I am assuming some are better than others? How do I use the best one?
Please help! Thank-you.

The Pipeline tab allows you to active and order any of the available effects.

The Common tab allows you to modify general values and attributes.

The BasicFX tab (suite) allows you to modify values of general effects such as splitscreen or UI-Mask.

The CustomFX tab (suite) contains not yet categorized or individual effects from various developers.

The following tabs (suites) devide all remaining effects into functional and responsibility domains. They are distinct suites so non is worse or better. Alternatives to one effect are usually found in the same domain/suite.

For more information, read up HERE

Okay, thank-you very much. That explains most of it for me.
But what about the second box, to the right of where I locate my game file?

I guess you mean the global preset category:

You can store your configuration either for a specific game profile when hitting apply while having that game profile selected in the app profile category or your store your configuration in unbound/global presets in the global preset category.

What are the benefitis of additional global presets compared to only using game profiles? While game profiles will be deleted with deleting the game profile, global presets will remain independet of any game profile. Further, global presets are meant to be shared between users. For your comfort, the mediator creates with the preset a theLazy.Net file which can be uploaded to theLazy.Net and found and automatically applied by other users with the crawling function which is build in the mediator.

Okay, thank-you. So it is really just for sharing your profiles? I do not entirely understand, but I do not think I will be using it.
Unless of course it would be used to allow people to create better ones? I don't know. But, thank-you for explaining.

Yes, its mainly for sharing or as a backup for yourself.

Imagine you have 3 configurations you like and you want to switch between them depending in which mood you are. Or you want to just keep your last configuration before working on a new one. Thats the way to go!